Quick questions on Intermolecular forces and properties of covalent molecular substances (QCE Chemistry Unit 1)

Dispersion forces (also called London forces or instantaneous dipole-induced dipole forces). Present between all molecules, polar or non-polar. Caused by random fluctuations in electron density that create an instantaneous dipole; this induces a dipole in a neighbouring molecule; the two attract. Always present, but they are the only intermolecular force in non-polar substances.

1. A hydrogen atom covalently bonded to N, O or F (the donor). 2. A lone pair on N, O or F in a neighbouring molecule (the acceptor).

Melting point and boiling point. Higher with stronger intermolecular forces. The kinetic energy required to overcome the attractions is proportional to their strength. When comparing substances:

Notice the periodic-table dip down to CH_4 and the spike at H_2O. Water boils higher than HF and NH_3 because each H_2O can engage in 4 hydrogen bonds on average, while HF has 1 H-F donor and NH_3 has 3 donors but only 1 acceptor lone pair.

Present between all molecules, polar or non-polar. Caused by random fluctuations in electron density that create an instantaneous dipole; this induces a dipole in a neighbouring molecule; the two attract. Always present, but they are the only intermolecular force in non-polar substances.

Present in polar molecules. The delta+ end of one molecule attracts the delta- end of a neighbouring molecule. Stronger than dispersion forces of comparable mass, but typically weaker than hydrogen bonding.

A special, particularly strong dipole-dipole attraction that occurs when H is bonded directly to N, O or F (the three small, highly electronegative atoms). The H is so strongly delta+ that it attracts a lone pair on N, O or F of a neighbouring molecule. Roughly 5 to 10 times stronger than ordinary dipole-dipole attraction.

Higher with stronger intermolecular forces. The kinetic energy required to overcome the attractions is proportional to their strength. When comparing substances:

"Like dissolves like". Polar solvents (water, ethanol) dissolve polar and ionic solutes; non-polar solvents (hexane, oil) dissolve non-polar solutes. The principle is that the solute-solvent interactions must be of comparable strength to the solute-solute and solvent-solvent interactions.

Higher with stronger intermolecular forces (molecules resist sliding past each other). Glycerol (three O-H groups, extensive hydrogen bonding) is much more viscous than water; water is much more viscous than hexane (only dispersion).

Higher with stronger intermolecular forces (the surface molecules are pulled inward more strongly). Water has high surface tension because of hydrogen bonding; hence water beads up on a non-polar surface.

Lower with stronger intermolecular forces. Substances with strong intermolecular forces have fewer molecules escaping to the vapour phase at any given temperature.

They are attractions between molecules, not bonds within molecules. Reserve "bond" for ionic, covalent and metallic interactions.

Polar molecules have dispersion forces too. The total intermolecular force is the sum of all applicable types.

HCl, H_2S, CHCl_3 do not have hydrogen bonding. They are polar (dipole-dipole) but the H is not bonded to N, O or F.